Yeah that is a problem with the tank. The way the tank is built, I have to have the water almost to the rim for the filter to work. Bettaboy thinks that is wonderful, he gets right the to the rim of the tank, checking it out. Since the water is so high, the thermometer sits outside the top of the tank. This stops me from putting the top back on the tank as the head of the thermometer doesn't fit in the hole they left for cords. Very poorly designed tank.
I am thinking maybe I am gong to take the filter out. It will mean more water changes but I will be able to lower the waterline, put the thermometer further into the tank and put the lid on it. I liked having the filter, Bettaboy there learned to surf the waves and it gave me extra time on water changes...but it is what it is.